Pile Design Based on Cone Penetration Test Results

Abstract

The bearing capacity of piles consists of both base resistance and side resistance. The side resistance of piles is in most bases fully mobilized well before the maximum base resistance is reached. As the side resistance is mobilized early in the loading process, the determination of pile base resistance is a key element of pile design. Static cone penetration is well related to the pile loading process, since it is performed quasi-statically and resembles a scaled-down pile load test. In order to take advantage of the cone penetration test for pile design, load-settlement curves of axially loaded piles bearing in sand were developed in terms of normalized base resistance versus relative settlement. Although the limit state design concept for pile design has been used mostly with respect to either 5% or 10% relative settlement, the normalized load-settlement curves obtained in this study allow determination of pile base resistance at any relative settlement level within the 0-20% range. The normalized base resistance for both non-displacement and displacement piles were addressed. In order to obtain the pile base load-settlement relationship, a 3-D nonlinear elastic-plastic constitutive model was used in finite element analyses. The 3-D nonlinear elastic-plastic constitutive model takes advantage of the intrinsic and state soil variables that can be uniquely determined for a given soil type and condition. A series of calibration chamber tests were modeled and analyzed using the finite element approach with the 3-D nonlinear elastic-plastic stress-strain model. The predicted load-settlement curves showed good agreement with measured load-settlement curves. Calibration chamber size effects were also investigated for different relative densities and boundary conditions using the finite element analysis. The value of the normalized base resistance was not a constant, varying as a function of the relative density, the confining stress, and the coefficient of lateral earth pressure at rest. The effect of relative density on the normalized base resistance was most significant, while that of the confining stress at the pile base level was small. At higher relative densities, the normalized base resistance was smaller (0.12-0.13 for 90% relative density) than at lower relative densities (0.19-0.2 for 30% relative density). The values of the normalized base resistance for displacement piles are higher than those for non-displacement piles, being typically in the 0.15-0.25 range for 5% relative settlement and in the 0.22-0.35 range for 10% relative settlement. The values of the normalized base resistance for silty sands are in the 0.12-0.17 range, depending on the relative density and the confining stress at the pile base level. The confining stress is another important factor that influences the value of the normalized base resistance for silty sands. For lower relative density, the value of the normalized base resistance decreases as the pile length increases while that for higher relative density increases.